In recent years, a request to an image forming apparatus for high image quality is getting higher. Besides, a color image forming apparatus becomes widespread. The color image forming apparatus, for example, an electrophotographic apparatus sequentially superposes toners of, for example, four colors of Y (Yellow), M (Magenta), C (Cyan) and K (Black) on a transfer target body and expresses a full-color image on the transfer target body. When a gradation property is changed between the colors of the respective toners even if only slightly, the tint of a final image is changed. It is important to keep the gradation property constant against the deterioration with age of the image forming apparatus or environmental change. Then, in order to keep the gradation property, various image quality maintaining control methods are investigated.
As a generally used image quality maintaining control method for keeping the gradation property, the following method can be mentioned. First, a scanner or the like as an original document reading apparatus reads a specific pattern, such as a gradation pattern, printed on a paper or the like as a print member. The scanner or the like holds the read information as reference density. After a certain period of time, an image forming section prints the specific pattern such as the gradation pattern. The scanner or the like again reads the specific pattern. An image processing section compares the read information with the reference density and determines a correction coefficient. The image processing section causes the correction coefficient to be fed back to the image forming section.
However, in the above method, when the image forming apparatus is used for a long period of time, there is a problem that even if the image processing section executes the image quality maintaining control, the read density does not reach the reference density. When the image processing section executes the image quality maintaining control, the scanner or the like must always be used. There is a problem that it takes much time to execute the image quality maintaining control or handling by a person is complicated.
Then, a control method of an image processing apparatus is proposed in which a change in toner density value on a photoconductive drum from a reference density value is examined, a correction table is formed from the change amount, the correction table and a lookup table for converting a density signal into a laser output signal are combined into one table, and γ correction is performed (for example, JP-A-2000-238341).
JP-A-2000-238341 discloses a method described below. A first control system of an image processing apparatus performs an automatic gradation correction in which an image on a recording member recording a specified image is read, and a laser output relative to an image signal is controlled so that the gradation of the image signal coincides with the gradation of the image recorded on the recording member (Act 271). Immediately after that, a second control system of the image processing apparatus forms a patch on the photoconductive drum by the laser output controlled in the first control process, and detects the density value of the patch (Act 274).
Thereafter, for example, when the power source of the image forming apparatus is turned ON, the second control system detects the density value of the patch formed on the photoconductive drum at a specified timing (Act 277), and forms a correction LUT for controlling the laser output relative to the image signal so that the detected density value coincides with the reference density value (Act 278).
In the image processing apparatus disclosed in JP-A-2000-238341, after the second control system reads the patch, the correction table is formed, the laser output is controlled based on the correction table, and the reference density is determined. However, in the image processing apparatus, it takes much time to perform the image quality maintaining control operation as before. Besides, since the laser output is determined based on the correction table, in the patch pattern for each color formed when the reference density is formed, the input value varies at each time of correction. It can be inconvenient to obtain the gradation property.